Self-Assembly of Silver Clusters into One- and Two-Dimensional Structures and Highly Selective Methanol Sensing

The development of new materials for the design of sensitive and responsive sensors has become a crucial research direction. Here, two silver cluster-based polymers (Ag-CBPs), including one-dimensional {[Ag22(L1)8(CF3CO2)14](CH3OH)2}n chain and two-dimensional {[Ag12(L2)2(CO2CF3)14(H2O)4(AgCO2CF3)4](HNEt3)2}n film, are designed and used to simulate the human nose, an elegant sensor to smells, to distinguish organic solvents. We study the relationship between the atomic structures of Ag-CBPs determined by x-ray diffraction and the electrical properties in the presence of organic solvents (e.g., methanol and ethanol). The ligands, cations, and the ligated solvent molecules not only play an important role in the self-assembly process of Ag-CBP materials but also determine their physiochemical properties such as the sensing functionality.


Introduction
New functional materials are of great importance for the design of highly sensitive and high-precision sensors for chemicals.The selective, precise, and swift measurement of volatile alcohols is critical in various areas such as the food industry [1,2], occupational safety [3], and forensics [4,5].Toward this direction, the multidimensional nanoscale materials have garnered a lot of attention owing to their peculiarity in structures and properties, such as atomic/molecular thickness, optical transparency, and large surface areas [6,7].The fabrication of alcohol sensing devices is an important aspect for the application of these materials.For instance, graphene-based nanomaterials are considered as one of the frontiers of exploring the sensing materials [8][9][10][11].The change in conductivity of graphenebased materials upon the adsorption of gas molecules leads to the gas detection.Besides this, some other 2-dimensional (2D) materials including nanostructures based on metal oxides [12,13], nanoporous silicon [14], hybrid carbon-based nanostructures [15], metal organic frameworks [16], and hybrids of fiber optics with nanostructures [17] have also displayed great promise in sensing of alcohols.
Here, we present the synthesis of 2 novel silver cluster-based polymers (Ag-CBPs) with atomically precise structures, including {Ag 22 film (abbreviated as Ag 22 -CBP and Ag 16 -CBP, respectively), which are determined by single-crystal x-ray diffraction.Interestingly, they show different conductivity under an external voltage in the presence of different organic solvents, which is related to their structural differences.We further find that the charge transfer and the species of charge carriers have an important influence on the conductivity of Ag-CBPs.The different responses of Ag-CBPs to the variation of organic solvents hold promise in the design of sensitive sensors for distinguishing solvents like the nose to differentiate smells.

Synthesis of Ag 22 -CBP and Ag 16 -CBP
All the operations were carried out in dark.In general, 10 mg of Ag-L1 was dispersed into 5 ml of MeOH, followed by the addition of AgCO 2 CF 3 (88 mg, 0.4 mmol, dissolved in 5 ml of MeOH).A white suspension was generated.After 20 min later, filtration of the mixture gave a light-yellow solution, which was then exposed to ethyl ether for crystallization in the dark in a refrigerator.Transparent yellow block-like crystals of {Ag 22 (L1) 8 (CO 2 CF 3 ) 14 (CH 3 OH) 2 } n (denoted as Ag22-CBP) were obtained in a few weeks.{[Ag 12 (L2) 2 (CO 2 CF 3 ) 14 (H 2 O) 4 (AgCO 2 CF 3 ) 4 ](HNEt 3 ) 2 } n (Ag16-CBP) was obtained by a similar procedure.Briefly, 10 mg of Ag-L2 (dispersed in 5 ml of MeOH) was used in the preparation of Ag16-CBP.Light-yellow platelike crystals of Ag 16 -CBP were obtained after weeks in the dark in a refrigerator.The synthetic yields of Ag 22 -CBP and Ag 16 were 41% and 62% (based on consumption of AgCO 2 CF 3 ), respectively.

Sensor fabrication
For the preparation of the cluster thin-film sensor, 30 mg of the clusters was first dispersed in 5 ml of ethanol, and then 5 ml of HPMC aqueous solution (4 mg•ml −1 ) was added to adjust the viscosity.Next, 3 ml of cluster dispersion obtained from the previous step was dripped on PET and dried at 60 °C in an oven for 10 min.Finally, Cu wires were attached to the two ends of the film for connecting to the power supply in electrical mea surements.The film of Ag-CBP sensors was sprayed into solvents.The pure solvents of methanol, ethanol, acetone, and toluene in HPLC grade and ultrapure water (resistance, 18.2 MΩ•cm) were used for the dynamic response and recovery of Ag-CBP sensors.The response time was 8 s, and the recovery time was 14 s.The sensitivity could be calculated as relative capacitance change for = , where δ is relative current change I x and I 0 are measured currents when the sensor contacted with solvents and the initial current of free sensor (before dipping into solvents), respectively.
The conductivity of the samples in solution was determined by a DDS-307 conductivity meter.Crystal samples were dispersed into different solution and then filtrated with a pinhole membrane filter, generating saturated solutions to be tested.Before the measurement, the conductivity meter was calibrated by a standard solution of 1408 μS•cm −1 .

Atomic structure of Ag-CBPs
The cluster-based polymeric materials in this study with compositions of Ag 22 -CBP and Ag 16 -CBP were synthesized through a bottom-up synthetic strategy.Briefly, these polymeric materials were produced by the reaction and self-assembly of the corresponding silver precursors (Ag-L1 and Ag-L2) with AgCO 2 CF 3 , respectively (see the Supplementary Materials for details).Here, the alkynylate and thiolate ligands are selected to construct highly stable Ag-CBPs for their strong interaction and the flexible coordination between ligands and Ag atoms [31].
The compositions and atomic structures of the Ag-CBPs are determined by single-crystal x-ray diffraction.The Ag 22 -CBP crystallizes in the C2/c space group.The minimum asymmetry unit of Ag 22 -CBP is constructed by 4 components, in cluding 11 Ag atoms, 7 trifluoroacetate anions, 4 L1 anions, and a methanol molecule, as depicted in Fig. 1A.An asymmetry unit rotates by 180° around a C 2 axis forming the monomer of Ag 22 -CBP (Fig. 1B).Thus, a metal framework of 22 Ag atoms is furnished in the monomer, where the distances between the silver atoms range from 2.798(4) Å to 3.353(3) Å.The monomers connect with each other in a head to tail manner, giving rise to a 1D silver chain along the c axis (Fig. 1C) through Ag-Ag bonds [3.302(4) Å], Ag-O-C(CF 3 )-O-Ag and Ag-O trifluoroacetate -Ag motifs, and Ag-alkynylate bonds (Fig. S1), which resembles a millipede.The obtained silver chains are covered by L1 ligands and trifluoroacetate ions through various bonds.For the L1 ligands, both the terminal C≡C and C≡N groups are bonded to Ag atoms (Fig. S2).Three types of coordination modes of terminal C≡C group are observed: (a) the μ 4 -η 1 , η 1 , η 2 , η 2 mode (Fig. S3A), in which the C≡C group bonds to 2 Ag atoms via σ bonds and to another 2 Ag atoms through π bonds; (b) the μ 5 -η 1 , η 1 , η 1 η 2 , η 2 mode (Fig. S3B), in which the C≡C group links to 3 Ag atoms via σ bonds and to another 2 Ag atoms through π bonds; and (c) the μ 4 -η 1 , η 1 , η 1 , η 2 mode (Fig. S3C), in which the C≡C group is bonded to 3 Ag atoms via σ bonds and to another Ag atom through π bond.Only the C≡N-Ag σ bonding mode is detected between the C≡N group and Ag (Fig. S2).Besides, 3 coordination modes are observed between trifluoroacetate ions and Ag atoms, namely, μ 1 -O trifluoroacetate , μ 2 -O trifluoroacetate , and μ 3 -O trifluoroacetate bonding modes.Besides L1 ligands and trifluoroacetate ions, it is worth noting that the nonionized methanol molecules bind with Ag atoms through Ag-O methanol bonds [2.425(2) Å], which are shorter than the Ag-O trifluoroacetate bonds [2.688(5) Å] (Fig. 1A and Fig. S4).These obtained sil ver chains are discrete but bridged to each other by C≡C groups and C≡N groups of L1 ligands to form the final 3D structure of Ag 22 -CBP, as shown in Fig. 1D.
A 2D structure of Ag 16 -CBP in P-1 space group was obtained when the bidentate L1 ligand was replaced by the unidentate L2 ligand under the otherwise similar experimental conditions.As shown in Fig. S5,    clusters is ~3.572Å, beyond the limit of Ag-Ag bonds.Furthermore, the Ag 2 (CF 3 CO 2 ) 2 units (noted as Ag 2 , highlighted in green in Fig. 2 and Fig. S6B) serve as the linkers to connect neighboring Ag 12 clusters in the bc plane, i.e., the Ag 12 clusters interact with 4 neighboring Ag 12 clusters through 4 Ag 2 units by the (Ag 12 )-O triflouroacetate -(Ag 2 ) motifs (Fig. 2c) to form a 2D plate.Finally, the 2D plates are packed into the 3D structure through hydrogen bonds and Van der Waals forces between layers (Fig. 2D).

Sensing response to alcohol
Different from the common silver complexes [42], silver clusters [18,19] and even silver wires [22], here, we have obtained the silver frameworks, silver chains in Ag 22 -CBP, and silver plates in Ag 16 -CBP with unique and continuous features, which provide good channels for electrons to go through and prompt us to study their conductivity in the solid state.
To further confirm the structure uniformity and purity of the obtained Ag 22 -CBp and Ag 16 -CBP samples, we performed powder x-ray diffraction (PXRD) of Ag 22 -CBP and Ag 16 -CBP with their crystals and compared with simulated ones from the single-crystal structures as shown in Figs.S9 and S10.For both Ag 22 -CBP and Ag 16 -CBP, their experimental PXRD profiles match well with the simulated curves, respectively, indicating the high purity of crystal phases and structural consistency in Ag 22 -CBP and Ag 16 -CBP.Besides, we recorded and compared the PXRD patterns of the bulk crystal and powder samples after grinding.The data showed that the structures of Ag-CBPs were the same before and after the grinding (vide infra).
We investigated the sensing property of the 1D Ag 22 -CBP chains and 2D Ag 16 -CBP plates on a home-made setup in a clean room at the ambient temperature of ~24 °C.During the tests, the powder of Ag-CBPs was painted on a flexible Polyethylene terephthalate board, which was further connected to a SourceMeter 2450 through Cu wires linking to polymer film on PET board, forming a closed circle (Figs.S11 and S12).Initially, the films made of Ag 22 -CBP and Ag 16 -CBP were insulating under dry conditions (as the reference).Once organic solvents (including protic solvents: methanol and ethanol; and aprotic solvents: acetone and toluene) were sprayed on the films, the film conductivity changes, and the film becomes conducting for methanol and ethanol but still insulating for acetone, toluene, and ultrapure water (resistance of 18.2 MΩ•cm) (Fig. 3 and Fig. S13).As shown in Fig. 3A, the electric current turns to zero again with the removal of methanol or ethanol from the thin films, indicating that the protic organic solvents should interact with Ag 22 -CBP and Ag 16 -CBP to make them conductive.On the con trary, the current was almost zero when acetone, toluene, and ultrapure water were sprayed on the films (Fig. 3A and Fig. S13), which corroborates that the Ag 22 -CBP and Ag 16 -CBP cannot detect these solvents, as these solvents cannot make the Ag 22 -CBP and Ag 16 -CBP nanomaterials conductive.The limit of detection is also evaluated in the methanol/water mixtures with different ratios, as shown in Fig. S13.The relative capacitance change of the Ag 16 -CBP sensor for methanol can reach up to 5,000 for the mixture of methanol and water (v methanol /v water = 3:7), which is ~33-fold higher than the reported cellulose/graphene nanocomposite [43].
The different responses to organic solvents suggest that both Ag 22 -CBP and Ag 16 -CBP could serve as good sensors for the protic organic solvents (e.g., methanol and ethanol), like the nose to recognize different smells in air.In comparison, Ag 22 -CBP and Ag 16 -CBP nanomaterials showed excellent dynamic responses to the methanol and ethanol detection, which may be due to the robust nature of these Ag-CBP nanomaterials.It is worthy to note that the responding electric current in methanol sensing is 15-to 25-fold that of the ethanol detection, and the signal falling time for methanol is longer as well (Figs.S14 and S15), which may be related to the stronger interaction between methanol and Ag 22 -CBP or Ag 16 -CBP than that of ethanol.The small cavities in Ag 22 -CBP (2.93 Å × 4.95 Å) and Ag 16 -CBP (3 Å × 3 Å) formed by the surface ligands of silver frameworks are much more suitable for small molecules like methanol and water only.In contrast, larger molecules such as ethanol require larger cavities than 3 Å.Furthermore, the electric current intensity of the Ag 16 -CBP film (about 600 nA) is ~10-fold that of the Ag 22 -CBP film (~60 nA) with almost the same size under the identical experimental conditions, and the recovery time of Ag 16 -CBP film (~22 s) is much longer than that of Ag 22 -CBP film (~5 s) (Fig. 3B).All these results suggest that the Ag 16 -CBP film is much more sensitive to methanol than the Ag 22 -CBP film, which should be caused by the methanol absorption capability of the sensing material, for which the factors including the higher density of Ag atoms in the structural arrangement, richer hydrogen bonds, ion features, and the mass transfer in the 2D film of the Ag 16 -CBP should also be responsible.
To reveal how the conductivity of the CBP was affected by methanol, we studied the structural variation of CBP films with PXRD as shown in Fig. 4. Powders of Ag 22 -CBP and Ag 16 -CBP were made into films on a sample stage and tested.For Ag 22 -CBP, the powder sample presented more obvious signals at low angles than the bulk crystal samples, which is related to the size of sample and crystal face exposed.Ag 22 -CBP samples show the same profiles in bulk crystal and powder status, indicating that Ag 22 -CBP shares the same structures in powder and crystal.After methanol spray, a subtle decrement in the degree of crystallinity was detected, suggesting that partial crystalline Ag 22 -CBP maybe turns into amorphous in the presence of methanol although the structure of Ag 22 -CBP keeps inert.Besides, no further loss in the degree of crystallinity of Ag 22 -CBP film occurred even using more methanol, and the loss is irreversible.The situation for the 2D Ag 16 -CBP is more complicated.Some peaks in the 5° to 20° range in PXRD formed and disappeared besides the loss of degree of crystallinity.X-ray diffraction peaks at 6.6°, 7.94°, 8.2°, 10°, and 19.5° disappeared after methanol spray, and 4 new peaks were found at 6.4° to 7.5° simultaneously.These results indicate that the size/structure of Ag 16 -CBP has been affected by the sprayed methanol, which may be related to the 2D structure of Ag 16 -CBP.For example, the distance between layers and the relative position of layers may be affected by inducing methanol molecular.

Mechanism
The sensitivity of sensors to solvents is closely related to the conductivity of Ag-CBP materials with solvents.Compared with the silver chains (1D) (Fig. 5A) in Ag 22 -CBP, the silver planes (2D) (Fig. 5B) in Ag 16 -CBP are more convenient for electrons to transfer in solid state under the applied voltage, generating a current.Furthermore, the positive pyridine rings and [HNEt 3 ] + in Ag 16 -CBP are helpful for the transfer of carrier, beside the silver planes.In fact, the dry Ag-CBP powders are insulators since the silver chains in Ag 22 -CBP and the silver planes in Ag 16 -CBP are disconnected, like a mess of broken electric wires, and no current can be generated even when a voltage is applied.However, once the Ag-CBP powders become wet by some solvents (such as methanol and ethanol would bond with the Ag atoms via Ag-O interactions), numerous microelectrolytic tanks (deep colored areas in Fig. 5) are formed among the broken silver chains and planes in Ag-CBPs, where ions dissociate from the Ag-CBPs and work as carriers in the electrolyte solution.Therefore, the solvent sensitivity of sensors made of Ag-CBPs depends on the conductivity of microelectrolytic tanks among silver chains and planes in Ag-CBP nanomaterials, as the charge transfer in solids of Ag-CBPs is immune from solvents outside.
The saturated methanol solutions of Ag 22 -CBP and Ag 16 -CBP were used to simulate microelectrolytic tanks between silver chains (1D) in Ag 22 -CBP and the silver planes (2D) in Ag 16 -CBP, respectively.The conductivity of saturated methanol solution of Ag 16 -CBP was determined as 608 μS•cm − − could be disassociated from Ag 22 -CBP and serve as carriers in solution.(b) In the coordination modes of Ag, all the Ag + cations were linked to L1 ligands through strong σ and π bonds, which is hard for Ag + to disassociate, while the Ag 2 (CF 3 CO 2 ) 2 units are free to generate ions in Ag 16 -CBP except the Ag + ions being bonded to L2 ligand via Ag-S bond.(c) The ligated waters and [HNEt 3 ] + ions in Ag 16 -CBP could provide more H + ions, which are the fastest ionic carrier (in terms of mass transfer) in solution.All these structural factors determine the superior conductivity of Ag 16 -CBP in solution and more sensitive response to methanol.Furthermore, the conductivity of Ag-CBPs in ethanol was measured as ~50 μS•cm −1 , ~0.45 μS•cm −1 in acetone, and near 0 μS•cm −1 in toluene, which could well explain the weak electric response to ethanol and null to acetone and toluene.

Conclusion
In summary, we have designed and prepared 2 novel nanoclusterbased polymers that are composed of Ag nanoclusters linked with each other.Their crystal structures are determined by x-ray diffraction, which indicates that the coordination modes of ligands have an important effect on the self-assembly of cluster-based materials.Furthermore, the conductivity of the 2 polymers in the solid state could be altered by organic solvents.Interestingly, the Ag-CBPs exhibit a high electric response to methanol, suggesting that the cluster-based materials can be used to design sensitive sensors to detect organic solvents, especially methanol as demonstrated in this work.Finally, we have discussed the mechanistic insight into how the cluster-based materials work as a sensor in detecting organic solvents in the presence of ambient atmosphere.The synthesis and application of cluster-based materials enriches the application of metal nanoclusters.

Fig. 3 .
Fig. 3. Dynamic response and recovery characterization of Ag-CBP sensors.(A) Dynamic response and recovery curves of Ag 22 -CBP thin-film sensor under different organic solvents (protic solvents: methanol and ethanol; aprotic solvents: acetone and toluene).(B) Dynamic response and recovery curves of Ag 22 -CBP and Ag 16 -CBP films in the presence of methanol.All the sprays were measured at 0.1-V bias.

Fig. 4 .
Fig. 4. (A) PXRD of Ag 22 -CBP tested before and after the spray of methanol.(B) PXRD of Ag 16 -CBP tested before and after the spray of methanol.
1 , ~3-fold that of Ag 22 -CBP (228 μS•cm −1 ), indicating that the microelectrolytic tanks among silver planes in Ag 16 -CBP showed better conductivity than that among silver chains in Ag 22 -CBP.For the better conductivity of Ag 16 -CBP in solution, the following structural factors may be responsible: (a) The species of carriers (Ag + , CF 3 CO 2 − , [HNEt 3 ] + , and even H + or [H 3 O] + in Ag 16 -CBP) could serve as carriers in solution, but only Ag + and CF 3 CO 2